Smokebox saddle machining
Work on 2-8-0 4709’s new cylinders continues apace and is nearing completion in the Midlands.
One of the last, but highly important jobs is the smokebox saddle machining, which is being undertaken by Roach Precision Engineering in Brierley Hill.
In Swindon times, the machining of such an integral part of the cylinder block like the smokebox saddle curves, would probably have been carried out on a very large boring machine. Roach however, has a very modern machine shop and so it was no surprise that new technology supplanted the traditional technique for this project.
The Roach team chose to machine the large radius of the smokebox using a CNC milling machine, producing the curve by milling straight cuts from front to back of the smokebox curve and then moving around the circumference of the casting. This method is proving to work very effectively and is generating a perfect curve, ready for the smokebox, for which we will be seeking sponsors early in 2022.
This work has now been largely completed, leaving only a few holes to be drilled and tapped into the main cylinder block castings before they are completely finished. The remaining holes centres need to be transferred from those already existing in 4709’s extension frames.
However, some historically poor welds on the extension frames have been identified, which are currently being removed and re-engineered. Once complete, the frames will join the cylinders at Roach in order to transfer the mounting hole centres.
Even with Swindon’s standardisation masterplan, it is not advisable to depend on the existing hole centres being exactly to the works drawing. So, the holes will be programmed on one of Roach’s milling machines, to clean up and then precisely align the centres, which will then be transferred to the cylinder blocks.
Concurrently, we recently received a pattern for the piston valve liners from which four liners have now been cast and are also on route to Roach for machining. Once complete, all of the castings to complete the cylinder assembly, save the the piston heads and valves, will have been cast and await machining.
The piston valve liner castings were produced by following precisely the technique that Swindon always used and which was brought to our attention by 4709’s metallurgist, Andrew Cole.
The team is eagerly anticipating the return of the new cylinder blocks and machined extension frames back at Tyseley where our engineer Mike Solloway, will commence the final assembly of 4709’s front end – real progress in a year which started so badly for us all and a wonderful Christmas present for the project in general.
